The proportion of digital images requiring printing or display is growing with the increasing use of digital images in the home, office and on the internet. The originating equipment is typically a digital camera or scanner. In the case of a digital camera, the scene illumination may be less than ideal, resulting in a color balance error on the final image. In the case of a print scanner the color cast in the original print could give an unacceptable scanned version of the print. Even if the originating equipment provides an accurate ‘well balanced’ image any subsequent processing may introduce color casts that need correction.
There are a large number of white balance patents for digital and video cameras but they have the advantage of sampling the color of the light before any non-linear processing and subsequent clipping. U.S. Pat. No. 4,883,360 and U.S. Pat. No. 4,954,884 are typical for digital cameras and U.S. Pat. No. 4,638,350 is typical for video cameras.
There are also a large number of patents relating to color balance of photographic printers where they have to accommodate the large range of exposures on color negative film. Many make use of tone scale statistics within the image such as that taught in EP 1014695 and other more complex methods such as taught in U.S. Pat. No. 4,677,465 and U.S. Pat. No. 4,729,016. The consequence is that they have to be more aggressive than is needed for digital cameras resulting in the possibility of a greater number of color balance failures than would be the case with a restricted color balance range. All of these rely on being able to process accurate unclipped image data which is not available from digital images submitted for printing.
The problem with color balancing digital cameras is that clipped highlights and shadows remain neutral even when the mid-range image data requires color balancing. This invention aims to be able to take any digital image and correct it for color balance whilst maintaining the neutrality of any highlights or shadows.